This application claims the priority of German patent document 199 46 463.4, filed Sep. 28, 1999, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method and apparatus for automatic brake intervention control in an all-wheel-drive vehicle.
Automatic brake control systems are generally known and are used more and more frequently for improving driving safety and driving dynamics. A special problem concerning driving dynamics occurs, however, when such controls are included in all-wheel-drive vehicles. Specifically in rough terrain, individual wheels or several wheels may lose contact with the ground, to varying degrees. As a result increased wheel slip occurs at those wheels which lose contact, reducing traction, specifically during extreme uphill or downhill driving. This, in turn, impairs driving safety.
Previous automatic brake control systems have attempted to solve this problem by performing a brake intervention if an increased slip was detected at a wheel. However, this approach has the disadvantage that, in the case of an increased slip, particularly during uphill or downhill driving, traction is already reduced, so that the brake intervention is often too late.
One object of the present invention is to provide a method and apparatus of the initially mentioned type which ensures essentially a continuously optimal traction in the case of all-wheel-drive vehicles and also in rough terrain.
This and other objects and advantages are achieved by the automatic braking control according to the invention, in which wheel compression and wheel rebound movements (herein, generally called "wheel suspension travels"), the wheel reaction forces or values derived from the above-mentioned quantities, (such as the corresponding rebounding and force rates) supply information concerning the ground adherence of a particular wheel. By detecting and analyzing these quantities, a conclusion can be drawn concerning the ground adherence of an individual wheel or the development of the ground adherence. The latter quantity, in turn, allows a prediction concerning slip which is to be expected. If the slip to be expected is above a certain limit value, a brake intervention can be implemented, even though the actual slip at each individual wheel has not yet exceeded a critical value. Such brake intervention before or simultaneously with the occurrence of a critical slip value at a wheel ensures continuous traction, so that a safety gain is achieved.
The wheel suspension travels or the wheel reaction forces can be determined, for example, by means of sensors.
The wheel reaction forces can preferably also be determined from the wheel suspension travels. Sensors for the wheel suspension travels already exist in some vehicles so that their signals can also be used in a simple manner. If the signals already exist on a vehicle data bus and are provided by an existing automatic brake control device, no significant additional hardware is necessary in order to implement the method according to the invention. The invention thus provides a particularly cost-effective solution to the problem of wheel slip.
In order to draw a conclusion concerning the development of slip and ground adherence of an individual wheel, the compression and rebound rates or the time variations of the wheel reaction forces are of interest. These values can easily be determined from the wheel suspension travels and from the wheel reaction forces. In addition, input quantities which indicate uphill or downhill driving, or lateral acceleration can be supplied to the control device. When taking into account these quantities, a certain brake intervention can be determined according to the driving situation. In this case, interventions during uphill or downhill driving differ from those during cornering, circular driving or slalom driving. During circular driving or slalom driving, it is possible that both wheels of one vehicle side lift off, whereas, during uphill or downhill driving, either an individual wheel or diagonally arranged wheel may lose more or less ground contact. Corresponding to the driving situation and the other input information, such as steering angles, etc., automatic brake control strategies can be defined and filed in the control unit.